Composite boards composed of a laminate whose outer sheets or plates consist of fiber-reinforced synthetic resins with a supporting core sandwiched therebetween are widely used in the construction industry in place of steel sheets, wood boards and the like.
In such constructions, the supporting core or sandwiched layer is oftentimes constructed as a shear- and thrust-resistant core which sometimes is made, for example, of polyurethane foam, wood, or the like. One of the problems associated with fiber-reinforced synthetic resin layers is that they possess relatively high strength, to wit, strength which is superior to that of metal layers, only in the directions in which the reinforcing fibers are oriented. This, of course, means that for each direction of expected stress or load, correspondingly oriented fiber material has to be provided. From a practical point of view, this is customarily achieved by orienting the fiber material in the direction of the main stresses to be expected. Thus, for example, if the sheet is to be subjected to longitudinal, transverse and torsion stresses or loads, fiber-reinforcement in at least three directions corresponding to these stresses and calculated on a plane stress condition have to be provided. The desired strength and stability which is superior to those of metals, is not accomplished by the provision of a prior art shear- or thrust-resistant core, such as, for example, disclosed in German Offenlegungsschrift No. 1,779,024 in which the core consists of foam or a honeycomb structure.
A different proposal for sandwich structures of the kind with which this invention is concerned is disclosed in German Auslegeschrift No. 2,032,423. According to the teachings of this Auslegeschrift, reinforcing inserts for the fiber-reinforced synthetic resin sheets are proposed. The individual layers or sheets are stitched together by stitching threads which penetrate the individual sheets or layers so that a stitched structure is obtained. This construction is exceedingly cumbersome due to the required stitching procedure and manufacture is expensive. Moreover, the ultimate structure obtained is not shear- and thrust-resistant.
Moreover, the prior art constructions, including those of the German Offenlegungsschrift and the German Auslegeschrift referred to, are relatively heavy, a disadvantage which should be avoided for many fields of application, such as the aircraft and space industry.